CEDEC
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CEDEC, the inert free-draining footpath gravel supplied by CED Ltd.
These notes are intended to provide some insight to how CEDEC works and may be used. We do, however, recommend that the formal CEDEC laying instructions be adhered to at all times.
History
CEDEC was developed following a request during the construction of Canary Wharf for a footpath gravel that was free of limestone so as not to affect the pH of the soil around the trees. It was suggested that it should be something like the surface upon which the French play boules. Initially we were somewhat perplexed as to how such a material would work without the 'gluing' effect found in the British limestone footpath gravels then available and with which we were familiar. Nevertheless in France there were hard limestones used that did not "set". Two British aggregates (a silver-grey third has now been added to the original red and gold, all being quartzite and/or granite) were found to have the correct grading and shape profiles to pack efficiently and one was chosen for Cabot Square at Canary Wharf. It worked well and, not long after, other designers started to ask for the same material. Almost entirely through word of mouth CEDEC has developed into a standard footpath gravel widely used in public landscapes and sometimes in private gardens too.
Aspects of usage and application
In using such a material, it is useful to understand how it works. CEDEC is a crushed aggregate sized below 6mm. It has a lot of very fine particles included. The range of sizes and shapes is such that there are relatively few interstices and a fully compacted bulk density of up to 2 tonnes per m³ may be expected. CEDEC is placed upon a sub-base which must itself be porous. This sub-base does two things. It provides support to the CEDEC and allows water passing through the porous CEDEC to continue down into the ground. In this context it is necessary to appreciate that CEDEC and/or its sub-base should not be laid on concrete, clay or any impervious material.
As has been said earlier, CEDEC does not set, it packs. Therefore it remains capable of being moved about, whether by a heel or turning tyres. However, CEDEC is not harmed by this - there is nothing to break so it can always be put back in place. When laid initially, CEDEC should be rolled as indicated in the laying instructions. This gives a useful start to the compaction process that will be continued by the processes of rain and being walked upon. If the area is always dry, CEDEC is not suitable. If the footpath is untrodden for a significant period especially before it is first used, the material may appear to become slightly 'fluffy'. If this happens, it can be corrected by using the footpath normally. If the packing process has to be speeded up, then re-rolling as originally indicated may be undertaken.
The rolling process in the laying is not really essential to the production of an effective footpath and can be reduced or omitted if near trees. It will simply take longer for the CEDEC to pack. When it is fully packed, CEDEC will feel firm but still have a loose crumb on the surface.
Design matters
There are some matters to bear in mind when using CEDEC. Particles may get lodged in shoes, as can any grit, so care should be taken if using CEDEC near quality wood or stone floors. If the distance between the CEDEC and these floors is small, then shoes must be taken off or thoroughly cleaned.
CEDEC can migrate if continually encouraged in one direction - generally downhill. In such cases, some form of step will need to be introduced to lessen the slope. In essence CEDEC must be constrained but even so, it can be kicked onto adjoining hard paving surfaces (for example smooth concrete or stone) on which it is able to act like marbles and make one's footing rather unsatisfactory. We have on occasions suggested that some granite setts be introduced as a buffer between the CEDEC path and the smooth surfaces, these setts being laid with the CEDEC as the bedding and jointing medium for which it is well suited.
Maintenance
In addition to the need to correct any damage, this mainly consists of the removal of rubbish and cigarette ends - no suction cleaners of course. In time and depending on pollution by other materials, it may be necessary to make some holes by fork and, possibly, top up with a little fresh material.
Non-standard applications and related matters
In general, CEDEC seems to be very forgiving of maltreatment. We know of customers who have laid it 75mm deep instead of 50mm and even of it being laid 100mm deep on a membrane, with a layer of lightweight rounded aggregate underneath. This last was on a roof. The water flow between the lightweight aggregate particles was sufficient. We have customers who have used it as a driveway, contrary to our advice and knowing that with power steering, some problems could arise. Nevertheless on viewing the result some eight years later and after much use, we were disconcerted to see how well the CEDEC still functioned. We do not, however, recommend its use for traffic generally, though once well-compacted it will not be harmed in any significant fashion if it is driven over by a fire engine or articulated lorry. If it is roughed up, simply spread it back to its correct levels. It is, however, not tarmac. Many are the situations in which it can be used but not everywhere. It can be used by wheelchairs, but it is harder work than a hard smooth surface. It has, though, been used in a care environment.
CED Ltd
Issue Date : 30 December 2004
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